Picture: R-392 Front
INTRODUCTION
The R-392/URR is an HF receiver which when combined with the T-195/GRC-19 transmitter, it forms the GRC-19 vehicular radio set. Collins answered an RFQ (Request For Quote) in mid-1950 by assembling a team of engineers at Cedar Rapids, Iowa, where they designed and built the R-392 receiver. This was used with the T-195 transmitter in the Korean War. The receiver covers the frequency from 500 Khz to 32 Mhz, and can receive AM, CW, and FSK. There is one knob to tune the kilohertz frequency and this is displayed on a 3 digit Veedar Root type counter. There is another knob to tune the megahertz frequency, and this displays the frequency on a 2 digit Veedar Root counter. It is a super hetero dyne type and is triple conversion on the 8 lowest frequency bands, and double conversion on all the other bands. It has a third IF of 455 Khz. It contains 25 valves. The receiver runs from 28 volts DC at 3 Amps. It has no high voltage power supply. The valves use 28 volts on their plates. The valve heaters are 26 volt, 12 volt, or 6 volt. It is 11.5 inches high, 11 inches wide, 14.125 inches deep, and weighs 52 pounds. It is in a sturdy aluminium case, which is waterproof, and also capable of being dropped from an aircraft by parachute.
Picture: R-392 Front
CONTROLS
The controls are dominated by the 2 large knobs, labeled MEGACYCLES and KILOCYCLES. Above this is the dial window, showing the frequency readout, also in megacycles and kilocycles. At the top is the signal strength meter, labeled CARRIER LEVEL, and calibrated from 0 to 100 dB. To the left of the meter are the 2 antenna terminals, a BNC socket and a terminal, both labeled ANT. There is also an antenna trimming knob labeled ANT TRIM. To the right of the meter is the main function switch. This turns the radio on, and is labeled OFF, STAND BY (filaments on), and NORMAL. There are 3 other positions labeled LIMITER, NET, and SQ (squelch). To the extreme right is the BFO PITCH knob. In a line under the meter is the BFO ON and OFF knob. Then the AGC OFF, ON, and CAL switch. Then the RF GAIN SQUELCH THRESHold knob, and the AF GAIN knob. To the left of the dial and megacycles knob, are 2 sockets, for audio output. The microphone and PTT may also be operated from here. To the right of the dial and kilocycles knob, is the DIAL ZERO knob, the BAND WIDTH switch (8, 4, and 2 KC), and the DIAL LOCK below this. At the bottom left is the POWER INPUT connector that is normally plugged into the T-195 transmitter. Then there is a 5 Amp fuse (LINE), a 500 milli-Amp fuse (PLATE), and 2 spare fuses. Then there is the DIAL DIM switch, which can turn the dial lights off, or dimmed or on full. Finally there is a BNC connector to allow the IF signal to be connected to a TTY converter. This has a dust cap on it.
Picture: R-392 Controls
ELECTRICAL DESIGN
The antenna enters the radio through the BNC connector (from the transmitter) or the antenna terminal (for connection to a separate whip or long wire). It goes through the antenna earthing relay, then to the antenna tuned circuits. These circuits cover 0.5 to 32 mHz.
Picture: R-392 Block Diagram
There are 3 tuned circuits, each in their own shielded RF can. They are the antenna, the first RF and the second RF tuned circuits (from the front to the back). These RF cans each have a single coil, and single trimming capacitor. Each RF can has a slug that slides in and out of it to perform the tuning. The slug is connected to a horizontal bar (the slug rack), and there are 3 slugs on the slug rack. As the megacycles knob and the kilocycles knob are turned, a gearbox adjusts a cam, and the cam raises or lowers the slug rack, thus tuning the 3 coils.
These circuits have to cover the range 0.5 to 32 mhz. This range is divided into 6 bands, used to tune this range. Each of the 6 bands uses 3 tuned circuits, (each in its own RF can), so there are 18 RF cans. Each band has a slug rack, each with a cam, and all cams are connected to the gearbox. The 6 slug racks to the right hand side of the receiver are (right to left), 1-2 mHz, then 0.5-1 mHz, then 2-4 mHz, then 4-8 mHz, then 8-16 mHz, then 16-32 mHz at the left. The diagram shows them top to bottom.
Picture: R-392 Slug Rack Location (RF module)
Next to this are the 3 valves, used as the first RF amplifier, the second RF amplifier and the first mixer. The first RF amplifier is a 26A6 valve, and this has AGC control from a separate AGC rectifier. The second RF amplifier is also a 26A6 valve, and this also has AGC control, but from a different AGC rectifier.
On the bands 0.5 to 8 mHz (lowest 8 bands), the RF amplifier output goes to the first mixer, a 26C6 valve. The first crystal oscillator (6AJ5) injects a signal to produce a 9 to 18 mHz output. So the lowest 8 bands are all shifted up in frequency. The first crystal oscillator (in the crystal oscillator module) has 5 crystals (9, 8, 10, 12.6, 7 mHz), and 3 crystals are used a second time (8, 9, 10 mHz) for the next 3 bands. The 8 frequencies are applied to the cathode of the first mixer. The output from the mixer goes to 3 coils on the extreme left of the RF module (lowest in the figure). This has a slug rack that is driven by a cam from the gearbox. It tunes from 9 to 18 mHz.
Picture: R-392 Top View
The second mixer, a 26C6 valve, accepts input in the 8 to 32 mHz range. On the 0.5 to 8 mHz bands, the input to the second mixer is 9 to 18 mHz. On the 8 to 32 mHz bands, the signal bypasses the first mixer, and goes to the second mixer directly. The second crystal oscillator (6AJ5) injects a signal to produce a 3 to 2 mHz output. This is tuned by the second variable IF slug rack, at the rear of the RF module. It has 3 tuned circuits in the 3 to 2 mHz range. Since this circuit only has a 1 mHz tuning range, it is coupled to the 1 to 2 mHz slug rack, as this also has a 1 mHz tuning range. That is why the slugs racks are out of order.
Picture: R-392 Tuning Block Diagram
The second crystal oscillator is in the crystal oscillator module. It uses the 32 position switch to select one crystal for each 1 mHz band from the megacycles knob. The crystals are (in order) 12, 15, 6.2, 14, 8, 9, 10, 11 mHz. Then the 12 mHz crystal is used again. Then a 13 mHz crystal. Then 14, 15, 8, 8.5 (a new one), 9, 9.5 (a new one), 10, 10.5 (a new one), 11, 11.5 (a new one), 12, 12.5 (a new one), 13, 9, 14, 9.667 (a new one), 10, 10.333 (a new one), 10.667 (a new one), 11, 11.333 (a new one) mHz crystals are used again. So 18 crystals are used on the 32 bands. The second crystal oscillator output uses a tuned circuit in the plate, for each switch position. The output is applied to the second mixer cathode.
The 3 to 2 mHz output goes to a third mixer, a 26D6 valve. It is the PTO (Permeability Tuning Oscillator), covering a 1 mHz tuning range, and driven directly by the kilocycles knob. This has a tunable oscillator, over the range 3.455 to 2.455 mHz, to produce the 455 kHz IF output.
Picture: R-392 Bottom View
The IF amplifier consists of 6 valves (26A6) to produce the amplification required at 455 kHz. The first IF amplifier has control from the RF gain and squelch circuit. The first and second IF amplifiers have control from the AGC rectifier. The signal strength meter is in the cathode of the second IF amplifier. All the IF amplifiers have bandwidth control, by switching in 3 extra windings on each IF transformer.
The output from the IF amplifier goes to the detector (12AU7) and then to the noise limiter (12AU7). The noise limiter can be turned on and off. The BFO is injected into the detector. It also can be turned on and off and the pitch varied as well. The first audio amplifier (6AJ5) goes to a phase inverter, to drive the 26A7GT audio power amplifier. It then goes to the output sockets, for speaker or headphone connection.
There is an AGC amplifier (26A6) connected to the fifth IF amplifier output. This has 4 outputs. One output goes to the cathode follower (12AU7) connected to the BNC socket, to provide an RTTY connection. One output goes to the squelch rectifier (12AU7) and a squelch control valve (12AU7) which operates the squelch relay. One output goes to the AGC rectifier (12AU7) to control the first RF amplifier. One output goes to the second AGC rectifier (12AU7) to control the second RF amplifier and the two IF amplifiers.
There is a sidetone input from the transmitter, so that the transmitted signal can be heard.
There is a crystal calibrator using a 200 kHz crystal and a 26A6 valve. There is a multivibrator (12AU7) that divides by 2, to provide 100 kHz signals. There is then a harmonic filter (12AU7) and a “distorter” (12AU7) that produces 100 kHz signals over the frequency range 0.5 to 32 mHz. This is injected into the antenna circuit.
MECHANICAL DESIGN
The radio is in a cast aluminium case. The case is waterproof and has all the controls on the front panel. When the radio was opened, it was very clean inside. There is a main chassis inside. Between the front panel and the chassis, is an area of tuning gears and cables.
The chassis has modules on the top. The main module is the RF tuning module. It can be removed by taking off the front panel, unplugging the cables, and unscrewing the green screws.
This module has the gearbox on the front, which is driven by the megacycles knob and the kilocycle knob.
This gearbox drives the 6 cams on the back of the module, and these cams raise and lower the tuning slugs, so that the antenna, first RF coils and second RF coils are tuned. It also drives the 6 position switch so that the appropriate coils can be selected. These circuits have to cover the range 0.5 to 32 mHz, which is divided into 6 bands.
Picture: R-392 Tuning Cams (back view)
The gearbox also drives a 32 position switch, one position for each megacycle selected by the megacycles knob. This selects crystals in the other module on the top, the crystal oscillator module. There are 2 oscillators, each with its own set of crystals and tuned circuits. These oscillators are injected into the 2 mixers. The first and second variable IF tuning coil slugs are also driven from the gearbox.
The gearbox also drives the PTO to tune the kilocycles directly from the kilocycles knob.
At the same time, the Veedar Root displays are turned over, to match the received frequency.
Picture: R-392 Gearbox
The chassis has modules underneath. There is a U shaped module on the bottom, which surrounds the PTO oscillator. The U shaped module is actually 3 modules screwed together. The module across the back is the IF Amplifier module. The small module is the Calibrator module. The large module is the Audio Output module. These can be removed in one piece, by unscrewing the GREEN coloured screws, unplugging the cables, and loosening a shaft coupler. It is then easily lifted out. The PTO is the round can (with the RED label), in the middle. The PTO can be unscrewed and unplugged and easily removed.
RESTORATION
The NAMELATE was missing, so a new one was purchased and screwed in place.
The ANTENNA terminal had been replaced by a VHF coax socket. This was removed and the correct binding post was added.
The radio was powered up, but it was not working. One of the IF valves had failed, as it had an open circuit heater. I had no 26A6 types, so a new valve was purchased and plugged in. The radio now worked.
The IF amplifier was aligned first, and this was fairly simple. The RF stages were not aligned as the sensitivity was good. The crystals were not adjusted either.
The dial indication did not agree with the actual received frequency. It was correct at the 000 setting but 18 Kcs above, at the 999 dial indication. An attempt was made to correct the frequency offset by adjusting the mechanical dial, the coupling arrangement and the end stops. Attempts to adjust the shaft to the PTO also could not correct the offset. It could not be mechanically corrected. The PTO was removed from the chassis and powered up on the bench. The input frequency of the mixer is 3 to 2 mHz. The output is 455 kHz. The variable PTO frequency is needed to be tunable from 3.455 kHz to 2.455 kHz (the dial going from 000 to 999). The PTO frequency was measured with a frequency counter, and it was too low, and did not cover the 1 mHz range required.
The manual warns against opening the PTO. The large RED sticker on the PTO also warns against opening it. WARNING, SEALED UNIT, DO NOT REMOVE THIS COVER. I could see no other option. So I carefully removed PTO, undid the screws, removed the cover, and examined the insides. I put a frequency counter on the output, and screwed the rotating shaft from end to end. The slug went in and out of the coil. It moved along a rack which had various discs on it. These could be moved to adjust the linearity. I did not wish to disturb this. However, the frequency was too low over the whole range. When examining the circuit, there appears to be a main capacitor across the coil, in the resonance circuit, with two small capacitors also in parallel. The whole circuit was tuned by the movable slug. I measured the capacitors with a capacitance bridge. The 370 pF measured 373 pF. The two 10 pF capacitors measured 11 pF each. This was not enough to cause the problem. I chose one of the small capacitors, and disconnected it. The frequency shifted higher by and the range was very close to 1 mHz. It now appeared to track correctly over the whole range. I reassembled the PTO, put it back in circuit, and the dial reading was now correct.
Picture: PTO Internal View
Picture: R-392 PTO circuit
The CARRIER meter (signal strength) moved only a little. I looked at the circuit, and it appeared as though the AGC amplifier was not providing enough gain. There are several functions performed by this amplifier. The main one is developing the AGC voltage. I disconnected the cathode follower and squelch outputs, thinking they may be loading the amplifier. There was no change.
Picture: R-392 AGC amplifier circuit.
The AGC amplifier has a tuned circuit in the plate supply. The tuning appeared very broad, not as sharp as the IF transformers. I substituted a new coil, by swapping the BFO coil. They looked the same and appeared to be made like an RF choke dipped in something. No change. I disconnected the tuning capacitor and measured it on a capacitance bridge. This seemed to operate correctly, so I put it back in circuit. The parallel capacitor C633 measured correctly. I disconnected the 2 AGC rectifiers. No change. After a week or so, in desperation, I replaced the capacitor with a new one. Suddenly the tuning of C634 was now peaking, with a sharp peak and there was lots of gain. I measured the removed capacitor again. It had no leakage (measured with the Fluke digital multimeter), and the capacitance was correct. But it was faulty. It was a moulded capacitor, similar to a mica capacitor. It was tropicalised. It was probably just a sealed paper capacitor. So it was leaking at the higher voltage and lowering the Q of the tuned circuit. The 4 circuit loads were reconnected. The CARRIER meter was now working properly.
PERFORMANCE
The radio performs very well. The input to the 455 kHz IF amplifier is 19 uV for a 10dB signal to noise, very good. The RF sensitivity was measured at the centre of every band, for a signal to noise ratio of 10 dB. Also very good.
| FREQUENCY (MHz) | SENSITIVITY (uV) | 0.5 | 16 | 1.5 | 4.5 | 2.5 | 3.0 | 3.5 | 1.2 | 4.5 | 1.3 | 5.5 | 1.1 | 6.5 | 0.8 | 7.5 | 0.6 | 8.5 | 0.8 | 9.5 | 0.6 | 10.5 | 0.5 | 11.5 | 0.6 | 12.5 | 0.5 | 13.5 | 0.6 | 14.5 | 0.6 | 15.5 | 0.6 | 16.5 | 0.5 | 17.5 | 0.5 | 18.5 | 0.4 | 19.5 | 0.4 | 20.5 | 0.4 | 21.5 | 0.4 | 22.5 | 0.4 | 23.5 | 0.4 | 24.5 | 0.4 | 25.5 | 0.3 | 26.5 | 0.3 | 27.5 | 0.3 | 28.5 | 0.3 | 29.5 | 0.3 | 30.5 | 0.3 | 31.5 | 0.4 |
MANUAL
There is a section on jamming (section 26 in the manual). When being jammed, it suggests that operation should continue. That the bandwidth should be reduced from 8 to 4 or 2 kilocycles. That tuning slowly either side of the signal may help. That the AGC should be turned OFF, and adjust the RF gain, and/or the limiter. Turn the function to SQUELCH and attempt to control the interference. The antenna trimmer should be adjusted. If using CW or RTTY, then adjust the BFO. Try the antenna reoriented in a different direction. If all these fail, then change to a new frequency.
There is a section on the climate. It talks about special operating conditions in the arctic, in the tropics, and in the desert.
There is a section on special tools. Inside the cabinet are 2 tube pullers. There is a philips screw driver, and a Bristo wrench.
There is a section on synchronising the cams used for tuning. There is a table of the cam positions for each band.
CONCLUSION
The radio is very nicely made. It is very heavy, but easy to operate. The sensitivity is very good, and the frequency dial is accurate. The tuning is heavy though, and there is no fly wheel effect. The movement of the slug racks is lovely to watch, when tuning or changing bands.
The radio had been repaired in the past. One of the tube removal devices was missing. There was a home made cable clamp on the main cable.
The U shaped module, could be made as 3 modules, and be able to be removed independently.
The AGC trimmer is difficult to access, and if the design was changed such that it was mounted on the side of the chassis, then adjustment through the side would be easy, without having to remove the module.
The BFO adjustment could be made through the top of the transformer. As it is, the U shaped module has to be removed so that the BFO frequency can be adjusted.
Picture13: GRC-19 in use at Wyong NSW May 2022
REFERENCES
TM 11-858 R-392-URR Radio Receiver (1954)
Copyright
Ray Robinson VK2NO